The present invention generally relates to radiocommunication systems and, more specifically, to an apparatus and associated method for efficiently controlling power amplifiers within radio transmitters in cellular systems.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of rapid growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as to maintain high quality service and avoid rising prices.
Throughout the world, one important step in cellular systems is to change from analog to digital transmissions. Equally important is the choice of an effective digital transmission scheme for implementing the next generation of cellular technology. Furthermore, it is widely believed that the first generation of personal communication networks (PCN) (employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make and receive calls in the home, office, street, car, etc.), would be provided by the cellular carriers using the next generation digital cellular system infrastructure and the cellular frequencies. The key feature demanded in these new systems is increased traffic capacity.
Currently, channel access is most commonly achieved using frequency division multiple access (FDMA) and time division multiple access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal's transmission power is concentrated. Interference with adjacent channels is limited by the use of bandpass filters which only pass signal energy within the specified frequency band. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies.
In most TDMA systems, a channel consists of a time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. A given signal's energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, the portion of the interference from different relative signal strength levels is reduced.
Capacity in a TDMA system is increased by compressing the transmission signal into a shorter time slot. As a result, the information must be transmitted at a correspondingly faster bit rate which increases the amount of occupied spectrum proportionally.
With FDMA or TDMA systems, or a hybrid FDMA/TDMA system, it is desirable to avoid the case where two potentially interfering signals occupy the same frequency at the same time. In contrast, code division multiple access (CDMA) allows signals to overlap in both time and frequency. Thus, all CDMA signals share the same frequency spectrum. In either the frequency or the time domain, the multiple access signals appear to be on top of each other.
For all such systems, but especially CDMA systems, power control is an important technique for balancing the desire to provide an end user with a sufficiently strong signal while at the same time not causing too much interference to other users. Power amplifiers (PAs) are widely utilized in radio transmitters in order to amplify an unamplified RF signal to a predefined power level at which the RF signal is to be transmitted. The power level at which the RF signal is transmitted is normally set to one of several predefined power level increments which is based upon the power level of a received transmission at a base station. The more accurate the
in the amplification of these signals to the predefined power levels, the more efficient the transmitter operation.
One goal in power amplifier operation is to reduce the power dissipation that occurs. To reduce the power dissipation within a power amplifier, the voltage supplied to the amplifier is reduced via control techniques. However, when the supply voltage is reduced too much, the amplifier can be brought into a saturated state. The power amplifier would then act non-linearly, causing distortion of the output signal.
While there have been attempts to increase power amplifier efficiency and power output, they have not been fully successful. For example, U.S. Pat. No. 5,430,410 describes the use of an envelope detector to detect the output of a power amplifier. The output signal of the power amplifier is compared to the input side of a power amplifier. The difference between the output and input is used to control the bias level of the power amplifier in order to maintain linear operation of a saturated power amplifier and thereby reduce distortion. However, this document does not address increasing efficiency in a power amplifier before saturation.
Additionally, the abstract of SU 1417174 is believed to describe the power amplification of signals in radio communication devices which attempts to improve amplifier operation and reduce distortion by inserting a differential amplifier between a power amplifier's inverting input and the output of the power amplifier. The inputs into the differential amplifier are a peak detected value of the output of amplifier and the difference between the peak value of the amplifier and a changing voltage value. The effect of this circuit is to reduce distortion by adjusting the supply voltage to compensate for the change in the bias level of the preceding amplifier stage.
None of the present power amplifiers and associated control circuits known by Applicant operate to reduce the power supply voltage thereby initiating a reduction in power dissipation and overall temperature of the mobile unit, while at the same time avoiding saturating the amplifier. Additionally, the current consumption from the battery which powers a mobile employing a power amplifier is reduced, thereby improving its performance. Thus, there is a need for a system that can utilize the advantages provided by a non-saturated power amplifier which is highly efficient.